U.S. patent application number 14/258323 was filed with the patent office on 2015-10-22 for high conductivity rotor cage for line start permanent magnet motor.
This patent application is currently assigned to Baldor Electric Company. The applicant listed for this patent is Baldor Electric Company. Invention is credited to Richard J. Budzynski, Robert F. McElveen, Michael J. Melfi.
Application Number | 20150303847 14/258323 |
Document ID | / |
Family ID | 54322841 |
Filed Date | 2015-10-22 |
United States Patent
Application |
20150303847 |
Kind Code |
A1 |
McElveen; Robert F. ; et
al. |
October 22, 2015 |
High Conductivity Rotor Cage for Line Start Permanent Magnet
Motor
Abstract
A method for synchronizing a high inertial load with a
line-start synchronous motor involves providing a rotor core with
rotor bars being formed of a highly conductive material. In
accordance with one aspect of the method, a user is directed to
operatively couple a load to the motor and drive the load from
start to at least near synchronous speed during steady state
operation of the motor with the load coupled thereto. The load has
an inertia that is greater than an inertia associated with a load
driven by a like motor subjected to an equivalent range of starting
current but having rotor bars formed from a conductive material
having a conductivity lower than that the highly conductive
material.
Inventors: |
McElveen; Robert F.;
(Anderson, SC) ; Budzynski; Richard J.;
(Simpsonville, SC) ; Melfi; Michael J.;
(Richfield, OH) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Baldor Electric Company |
Fort Smith |
AR |
US |
|
|
Assignee: |
Baldor Electric Company
Fort Smith
AR
|
Family ID: |
54322841 |
Appl. No.: |
14/258323 |
Filed: |
April 22, 2014 |
Current U.S.
Class: |
318/400.41 |
Current CPC
Class: |
H02K 17/26 20130101;
H02K 17/165 20130101; H02P 1/50 20130101; H02K 2213/03 20130101;
H02K 1/2706 20130101; H02P 1/46 20130101; H02K 21/46 20130101 |
International
Class: |
H02P 6/22 20060101
H02P006/22; H02K 1/27 20060101 H02K001/27; H02P 6/08 20060101
H02P006/08; H02K 17/16 20060101 H02K017/16 |
Claims
1. A method comprising: providing a line-start synchronous motor,
wherein the motor comprises: a stator; a rotor core disposed within
the stator, the rotor core comprising: a plurality of permanent
magnets defining a magnet area; a plurality of rotor bars spaced
about the rotor core, the rotor bars defining a rotor bar area, the
rotor bars being formed of a conductive material having a first
conductivity; and end members disposed on axial opposite ends of
the rotor core, the end members being in electrical contact with
the rotor bars; directing a user to operatively couple a first load
to the motor and drive the first load from start to at least near
synchronous speed during steady state operation of the motor with
the first load coupled thereto; wherein the first load has an
inertia, the first load inertia is greater than an inertia of a
second load, the second load inertia is a maximum of which the
motor is capable of driving from start to near synchronous speed
during steady state operation of the motor with the second load
coupled thereto when the motor has rotor bars formed from a
conductive material having a conductivity lower than the first
conductivity and the motor is subjected to an equivalent range of
starting current.
2. The method of claim 1, wherein the rotor bars are a high
conductivity material.
3. The method of claim 2, wherein the high conductivity rotor bars
are cast.
4. The method of claim 1, wherein the magnets define a general axis
of magnetization of each pole of the rotor, edges of the magnet
slots that face the general axis of magnetization define a
saturation boundary area, and at least some of the rotor bars are
disposed in the saturation boundary area and spaced from the magnet
slots.
5. The method of claim 1, wherein the motor is configured to attain
synchronous speed after starting of the motor with the first load
operatively coupled thereto.
6. A method comprising: accessing a line-start synchronous motor,
wherein the motor comprises: a stator; and a rotor core disposed
within the stator, the rotor core being rotatable relative to the
stator about a center axis, the rotor core having an outer
diameter, the rotor core comprising: a plurality of generally like
laminations stacked end to end to form a contiguous rotor core,
each of the laminations having: a plurality of magnet slots being
spaced radially inward of the rotor outer diameter with an end of
the magnet slots being adjacent to the rotor outer diameter and
extending generally inward toward the rotor center axis, the magnet
slots having permanent magnets disposed therein, the magnets
defining a general axis of magnetization of each pole of the rotor,
edges of the magnet slots that face the general axis of
magnetization defining a saturation boundary area; and a plurality
of rotor bar slots spaced about the rotor core center axis, each of
the rotor bar slots being radially inward of the rotor outer
diameter with an end of the rotor bar slot being adjacent to the
rotor outer diameter, at least some of the rotor bar slots being
disposed in the saturation boundary area and being spaced from the
magnet slots; a conductive material with a first conductivity
disposed in the rotor bar slots; and end members disposed on axial
opposite ends of the rotor core, the end members being in
electrical contact with the conductive material; operatively
coupling a first load to the motor; and driving the first load from
start to at least near synchronous speed during steady state
operation of the motor with the first load coupled thereto; wherein
the first load has an inertia, the first load inertia is greater
than an inertia of a second load, the second load inertia is a
maximum of which the motor is capable of driving from start to near
synchronous speed during steady state operation of the motor with
the second load coupled thereto when the motor has rotor bars
formed from a conductive material having a conductivity lower than
the first conductivity and the motor is subjected to an equivalent
range of starting current.
7. The method of claim 6, wherein the motor attains synchronous
speed after starting of the motor.
8. The method of claim 6, wherein the first load attains
synchronous speed after starting of the motor.
9. The method of claim 6, wherein a high conductivity material is
disposed in the rotor bar slots.
10. The method of claim 6, wherein the conductive material disposed
in the rotor bar slots comprises fabricated bars.
11. A method comprising: accessing a line-start synchronous motor,
wherein the motor comprises: a stator; and a rotor core disposed
within the stator, the rotor core comprising a plurality of
permanent magnets, a plurality of rotor bars spaced about the rotor
core, and end members disposed on axial opposite ends of the rotor
core, the end members being in electrical contact with the rotor
bars, the rotor bars being formed of a conductive material having a
first conductivity; operatively coupling a first load to the motor;
and driving the first load from start to at least near synchronous
speed during steady state operation of the motor with the first
load coupled thereto; wherein the first load has an inertia, the
first load inertia is greater than an inertia of a second load, the
second load inertia is a maximum of which the motor is capable of
driving from start to near synchronous speed during steady state
operation of the motor with the second load coupled thereto when
the motor has rotor bars formed from a conductive material having a
conductivity lower than the first conductivity and the motor is
subjected to an equivalent range of starting current.
12. The method of claim 11, wherein the motor attains synchronous
speed after starting of the motor.
13. The method of claim 11, wherein the first load attains
synchronous speed after starting of the motor.
14. The method of claim 11, wherein the rotor bars are
fabricated.
15. A method comprising: providing a line-start synchronous motor,
wherein the motor comprises: a stator; and a rotor core disposed
within the stator, the rotor core comprising a plurality of
permanent magnets, a plurality of rotor bars spaced about the rotor
core, and end members disposed on axial opposite ends of the rotor
core, the end members being in electrical contact with the rotor
bars, the rotor bars being formed of a conductive material having a
first conductivity; and directing a user to operatively couple a
first load to the motor and drive the first load from start to at
least near synchronous speed during steady state operation of the
motor with the first load coupled thereto; wherein the first load
has an inertia, the first load inertia is greater than an inertia
of a second load, the second load inertia is a maximum of which the
motor is capable of driving from start to near synchronous speed
during steady state operation of the motor with the second load
coupled thereto when the motor has rotor bars formed from a
conductive material having a conductivity lower than the first
conductivity and the motor is subjected to an equivalent range of
starting current.
16. The method of claim 15, wherein the rotor bars are
fabricated.
17. The method of claim 15, wherein the rotor bars are formed from
highly conductive material.
18. The method of claim 15, wherein the magnets define a general
axis of magnetization of each pole of the rotor, edges of the
magnet slots that face the general axis of magnetization define a
saturation boundary area, and at least some of the rotor bars are
disposed in the saturation boundary area and spaced from the magnet
slots.
19. The method of claim 18, wherein the rotor bars are radially
inward but adjacent to an outer diameter of the rotor core.
20. The method of claim 15, wherein the motor is configured to
attain synchronous speed after starting of the motor with the first
load operatively coupled thereto.
Description
BACKGROUND
[0001] Synchronous motors, including line start, interior permanent
magnet (LSIPM) motors, are typically very efficient. A LSIPM motor
will produce torque to accelerate from zero speed when started
across the line, and then operate as a synchronous motor with no
rotor cage losses once fully up to synchronous speed. However,
synchronous motors have limited capability to pull into synchronism
loads that have a high torque or high inertia. For certain
applications, it is necessary for a LSIPM to demonstrate
satisfactory starting performance in addition to the steady-state
performance. For a LSIPM motor, this includes more than just
meeting rated starting current and starting torque during the
asynchronous period of acceleration as would be the case for an
induction motor. The LSIPM motor must also be able to pull a load
into synchronism and achieve normal steady state operation. Both
load torque and load inertia are considerations whether a specific
LSIPM motor will be able to successfully start and synchronize a
load. Accordingly, the benefits in efficiency gains and energy
savings ordinarily associated with synchronous motors are not
typically achieved in applications having loads with high inertia
and/or high torque characteristics. In the past, an inverter has
been used with synchronous motors in such applications to power the
motor during starting. However, an inverter adds substantial costs
and degrades system efficiency.
[0002] To achieve the steady state benefits of efficiency provided
by synchronous motors, and reduce limitations during start-up,
rotor end rings and rotor bars may be designed to improve the
ability of a motor to synchronize loads with higher torque and/or
inertia requirements compared with similar motors having
conventional end ring and rotor bar designs. The rotor end rings
and rotor bars may be configured to reduce full load asynchronous
slip by decreasing rotor resistance during start-up. While a
decrease in rotor resistance may theoretically be achieved using
induction motor principles (i.e., by increasing the total cross
sectional area of the rotor bars forming the starting cage),
increasing the area of the rotor bars has a negative impact on the
starting and full load operating performance of the motor. For
instance, increased rotor bar area results in increased flux
density in the rotor and lower power factor, higher current, and
more losses at full load, and higher locked rotor current at
starting.
SUMMARY
[0003] This disclosure is directed to employing an end ring which
has a larger cross sectional area than would typically be used for
the given bar area in order to reduce the asynchronous slip and
improve load synchronization capability while not impacting the
starting current or full load performance of the machine running as
a synchronous motor. This disclosure is also directed to employing
a rotor cage formed from materials with favorable conductive
properties in order to reduce the asynchronous slip and improve
load synchronization capability while not impacting the starting
current or full load performance of the machine running as a
synchronous motor.
BRIEF DESCRIPTION OF THE DRAWINGS
[0004] FIG. 1 is perspective view of an LSIPM.
[0005] FIG. 2 is a partial cross-section view of an electric motor
taken from lines 2-2 of FIG. 1.
[0006] FIGS. 3-6 show illustrative embodiments of laminations used
in a rotor of the motor of FIG. 1.
[0007] FIG. 7 is an enlarged view of an end ring.
[0008] FIG. 8 is a cross-sectional view of the end ring taken from
lines 8-8 of FIG. 7.
[0009] FIG. 9 is a chart showing the slip and load inertia for a
given end ring width for a LSIPM.
DETAILED DESCRIPTION
[0010] FIG. 1 illustrates an exemplary LSIPM 10. The exemplary
motor 10 comprises a frame 12 capped at each end by drive and
opposite drive end caps 14,16, respectively. The frame 12 and the
drive and opposite drive end caps 14,16 cooperate to form the
enclosure or motor housing for the motor 10. The frame 12 and the
drive and opposite drive end caps 14,16 may be formed of any number
of materials, such as steel, aluminum, or any other suitable
structural material. The drive and opposite drive end caps 14,16
may include mounting and transportation features, such as the
illustrated mounting feet 18 and eyehooks 20.
[0011] To induce rotation of the rotor, current is routed through
stator windings disposed in the stator. (See FIG. 2). Stator
windings are electrically interconnected to form groups. The
arrangement of the windings in the stator core defines the phases
associated with the LSIPM. The stator windings are further coupled
to terminal leads (not shown), which electronically connect the
stator windings to an external power source (not shown), such as
480 VAC three-phrase power or 110 VAC single-phase power. A conduit
box 24 houses the electrical connection between the terminal leads
and the external power source. The conduit box 24 comprises a metal
or plastic material, and advantageously, provides access to certain
electrical components of the motor 10. Routing electrical current
from its external power source through the stator windings produces
a magnetic field that induces rotation of the rotor. A rotor shaft
26 coupled to the rotor rotates in conjunction with the rotor. That
is, rotation of the rotor translates into a corresponding rotation
of the rotor shaft 26. The rotor shaft may be coupled to any number
of drive machine elements, thereby transmitting torque to the given
drive machine element. By way of example, machines such as pumps,
compressors, fans, conveyors, and so forth, may harness the
rotational motion of the rotor shaft 26 for operation.
[0012] FIG. 2 is a partial cross-section view of the motor 10 of
FIG. 1 along plane 2-2. To simplify the discussion, only the top
portion of the motor 10 is shown, as the structure of the motor 10
is essentially mirrored along its centerline. As discussed above,
the frame 12 and the drive and opposite drive end caps 14,16
cooperate to form an enclosure or motor housing for the motor 10.
Within the enclosure or motor housing resides a plurality of stator
laminations 30 juxtaposed and aligned with respect to one another
to form a lamination stack, such as a contiguous stator core 32. In
the exemplary motor 10, the stator laminations 30 are substantially
identical to one another, and each stator lamination 30 includes
features that cooperate with adjacent laminations to form
cumulative features for the contiguous stator core 32. For example,
each stator lamination 30 includes a central aperture that
cooperates with the central aperture of adjacent stator laminations
to form a rotor chamber 34 that extends the length of the stator
core 32 and that is sized to receive a rotor. Additionally, each
stator lamination 30 includes a plurality of stator slots disposed
circumferentially about the central aperture. The stator slots
cooperate to receive one or more stator windings 36, which are
illustrated as coil ends in FIG. 2, that extend the length of the
stator core 32. As described in more detail below, upon start-up,
the stator winding is energizable with an alternating voltage to
establish a rotating primary field that co-acts with the rotor bars
of the rotor to start the rotor under induction motor
principles.
[0013] In the exemplary motor 10, a rotor assembly 40 resides
within the rotor chamber 34. Similar to the stator core 32, the
rotor assembly 40 comprises a plurality of rotor laminations 42
aligned and adjacently placed with respect to one another. Thus,
the rotor laminations 42 cooperate to form a contiguous rotor core
44. When assembled, the rotor laminations 42 cooperate to form a
shaft chamber that extends through the center of the rotor core 44
and that is configured to receive the rotor shaft 26 therethrough.
The rotor shaft 26 is secured with respect to the rotor core 44
such that the rotor core 44 and the rotor shaft 26 rotate as a
single entity about a rotor center axis 45.
[0014] The exemplary rotor assembly 40 also includes electrically
conductive members, such as rotor bars 48, disposed in the rotor
core 44 electrically connected to rotor end rings or end members 46
to form the starting cage. The end rings or end members 46, which
are disposed on opposite ends of the rotor core 44 are generally
circular in cross-section and have an outer diameter that generally
approximates the diameter of the rotor laminations 42. The rotor
bars 48 in cooperation with the end rings 46 form at least one
closed electrical pathway for induced current within the rotor 40.
Accordingly, the rotor bars 48 and the end rings 46 comprise
materials having good electrical conductivity, such as copper
alloys as described below. Additional detail of the rotor bars and
the end rings will be described in greater detail below.
[0015] To support the rotor assembly 40, the exemplary motor 10
includes drive and opposite drive bearing sets 50,52, respectively,
that are secured to the rotor shaft 26 and that facilitate rotation
of the rotor assembly 40 within the stationary stator core 32.
During operation of the motor 10, the bearing sets 50,52 transfer
the radial and thrust loads produced by the rotor assembly 40 to
the motor housing. Each bearing set 50,52 includes an inner race 54
disposed circumferentially about the rotor shaft 26. The tight fit
between the inner race 54 and the rotor shaft 26 causes the inner
race 54 to rotate in conjunction with the rotor shaft 26. Each
bearing set 50,52 also includes an outer race 56 and rotational
elements 58, which are disposed between the inner and outer races
54,56. The rotational elements 58 facilitate rotation of the inner
races 54 while the outer races 56 remain stationary and mounted
with respect to the drive and opposite drive end caps 14,16. Thus,
the bearing sets 50,52 facilitate rotation of the rotor assembly 40
while supporting the rotor assembly 40 within the motor housing,
i.e., the frame 12 and the drive and opposite drive end caps 14,16.
To reduce the coefficient of friction between the races 54,56 and
the rotational elements 58, the bearing sets 50,52 are coated with
a lubricant. Although the drawings show the bearing sets 50,52 with
balls as rotational elements, the bearing sets may be other
constructions, such as sleeve bearings, pins bearings, roller
bearings, etc.
[0016] FIGS. 3-6 provide further detail of illustrative embodiments
of the rotor laminations 42. Each rotor lamination 42 has a
generally circular cross-section and is formed of a magnetic
material, such as electrical steel. Extending from end-to-end,
i.e., transverse to the cross-section, each lamination 42 includes
features that, when aligned with adjacent laminations 42, form
cumulative features that extend axially through the rotor core 44.
For example, each exemplary rotor lamination 42 has a circular
shaft aperture 62 located in the center of the lamination 42. The
shaft apertures 62 of adjacent laminations 42 cooperate to form a
shaft chamber configured to receive the rotor shaft 26 (see FIG. 2)
therethrough. The rotor core has an outer diameter ("D.sub.r").
[0017] Additionally, each lamination 42 includes a series of rotor
bar slots 64 that are arranged at positions about the lamination
such that when assembled, the rotor bar slots cooperate to form
channels for the rotor bars that extend through the rotor core 44.
The rotor bar slots are spaced radially inward from the rotor outer
diameter (D.sub.r). As shown in the drawings, each of the rotor bar
slots may extend radially outward to generally the same radial
position relative to the rotor outer diameter (D.sub.r), or one or
more rotor bar slots may extend radially outward and terminate at
different radial distances relative to the outer diameter
(D.sub.r), depending upon the application. The rotor bars 48 may
present the same shape as the rotor bar slots 64 to provide a tight
fit for the rotor bars 48 within the rotor channels. The rotor bars
may be manufactured with tight tolerances between the rotor bars 48
and the rotor bar slots, for instance, for a fabricated/swaged
rotor bar design.
[0018] Additionally, the rotor laminations 42 include magnet slots
70. Magnets 72 may be disposed in the magnet slots in various ways
to form poles for the rotor. The magnet slots may be arranged so
the magnets are in a single layer or multi-layers. The magnet slots
may also be arranged so the magnets form a conventional "v"- or
"u"-shape, or an inverted "v"- or "u"-shape. There may be only one
magnet per slot or multiple magnets per slot. The magnets may be
magnetized in a generally radial direction to establish alternately
inwardly and outwardly disposed north and south poles on adjacent
magnets. This means that adjacent magnets cooperate to establish
alternate north and south poles on the periphery of the rotor. The
rotor may be constructed with any even number of poles. An
exemplary lamination for a two pole motor is shown in FIG. 3, and
exemplary laminations for a four pole motor are shown in FIGS. 4-6.
As shown in the drawings by example and not in any limiting sense,
the magnets may establish a direct axis as indicated by reference
character 80 and a quadrature axis as indicated by reference
character 82. The magnets define a general axis of magnetization
(north or south pole) on the periphery of the rotor. The edges of
the magnet slots facing the general axis of magnetization, which
are radially outward from the magnets, establish a generally
arcuate saturation boundary area as indicated by reference
characters 84a,84b. In cases, where a magnet is disposed in the
magnet slot, the edges of the magnet slots facing the general axis
of magnetization and the edges of the magnets will be the same.
FIGS. 3 and 6 show embodiments where there is a gap 85 between the
permanent magnets in the magnet slots. In a multi-layer arrangement
such as shown in FIG. 6, the saturation boundary area is defined by
the magnet slots that are nested radially outward the farthest.
[0019] In each of the designs of the laminations shown in FIGS.
3-6, the magnet slots 70 extend to the peripheral edge of the rotor
such that an end of the magnet slot is adjacent the peripheral
edge. One or more of the magnet slots may have its radially outward
end at generally the same radial position relative to the rotor
outer diameter (D.sub.r) and the rotor bar slots as shown in the
drawings, or one or more magnet slots may extend radially outward
and terminate at different distances relative to each other and/or
the rotor bar slots, depending upon the application. The magnets 72
disposed in the magnet slots have a smaller longitudinal length in
the direction of the magnet slots than the magnet slots such that
the magnet when installed in the magnet slot forms a magnet slot
aperture 86 between the end of the permanent magnet and the magnet
slot. The magnet slot aperture may be filled with conductive
material to form additional rotor bars that are also connected to
the end members 46.
[0020] The rotor bars 48 forming the starting cage may have a
different size, shape, and spacing from rotor bars found in a
machine having a uniform cage. Additionally, the rotor bar slots 64
may be distributed about the rotor in a manner that is asymmetric
rather than evenly distributed, i.e., asymmetric rather than
equiangularly spaced, around the outer edge of the lamination
surface. Additionally, the rotor bar slots may have an arbitrary
shape. The laminations may be stacked off-set to one another such
that the rotor bar in the slot has a helix relative to the rotor
axis of rotation. Additionally, a rotor bar slot 90 may be provided
to align with the quadrature axis 82. The rotor bar slot 90 of the
quadrature axis may have a geometry which matches at least one of
the rotor bar slots aligned with the direct axis 80. Although some
of the drawings show a plurality of rotor bar slots in the direct
axis and one rotor bar slot in the quadrature axis, other
variations may be used. The rotor bar area ("BA") is the cumulative
area of all of the rotor bar slots in a lamination that are
intended to be filled with conductive material, including areas
between magnets in the magnet slots, and including rotor bar slots
provided in the quadrature axis and outside of the saturation
boundary area.
[0021] The laminations shown in FIGS. 3-6 are configured to
optimize paths for flux over a range of conditions including at
rated load. In each of the laminations shown in FIGS. 3-6, the
arrangement of the starting cage of the rotor bars and the magnets
allows for passage of rotor flux under a wide range of loads and
operating conditions. With each of the exemplar embodiments of
FIGS. 3-6, the distance between the rotor bar slots disposed in the
saturation boundary area 84a,84b and the magnet slots is selected
so that each rotor bar slot in the saturation boundary area may be
positioned away from an adjacent magnet slot by a distance that
equals or exceeds about four percent (4%) of the pole pitch.
According to another aspect of the present disclosure, the closest
approach distance of any one of the rotor bar slots in the
saturation boundary area to an adjacent magnet slot may be about
equal or exceed four percent of the pole pitch. The closest
approach distance is referred to hereinafter as ("D.sub.rb-m") and
is defined by the equation ("D.sub.rb-m").gtoreq.0.04.times.("pp").
The pole pitch for the machine ("pp") may be defined by the
equation ("pp")={("D.sub.R").times.(.pi.)}/("P"), where "D.sub.R"
is the diameter of the rotor and ("P") is the number of poles for
the machine as defined by the number of groups of permanent
magnets. One or more of the rotor bar slots in the saturation
boundary area may be arranged to maintain this parameter relative
to an adjacent magnet slot. Rotor bar slots outside of the
saturation boundary area, for instance, rotor bar slots 90
generally aligned with the quadrature axis 82, may also be
positioned to maintain this parameter relative to an adjacent
magnet slot.
[0022] In the rotor designs shown in FIGS. 3-6, at least one of the
rotor bar slots 64 in the saturation boundary area has a radial
interior edge 92 which conforms generally to a side of the magnet
72 in the adjacent magnet slot 70. FIGS. 3-6 show the magnet
arranged in the magnet slot in various configurations. In each
example, the interior radial edge of one or more of the rotor bar
slots 64 in the saturation boundary area has a geometry which
generally matches the geometry of the magnet adjacent to the rotor
bar slot. One or more of the rotor bar slots in the saturation
boundary area may be formed to have a radial inward edge which
defines a reference plane generally parallel to the adjacent
magnet. In this way, one or more of the rotor bar slots may have a
distance to the adjacent magnet slot that meets or exceeds the four
percent (4%) of the pole pitch ("pp"). Rotor bar slots outside of
the saturation boundary area, for instance, rotor bar slots 90
generally aligned with the quadrature axis 82, may also be shaped
in a similar manner to maintain this parameter.
[0023] Referring to FIGS. 7 and 8, each of the end rings 46
comprises an annular disk with generally flat faces 100,102 and an
outer diameter surface 104 and an inner diameter surface 106. The
end ring outer diameter surface 104 has an outer diameter ("OD")
with a dimension that generally equal to the rotor core outer
diameter. The end ring inner diameter surface 106 has an inner
diameter ("ID") with a dimension that generally corresponds to the
rotor shaft outer diameter. The inner face 100 abuts the
laminations 40, and the outer face 102 may have features allowing
the rotor to be balanced (i.e., drilled holes). The inner and outer
faces 100,102 may be separated by a width ("w"). The outer and
inner diameter surfaces may be separated by a height ("h") which is
equal to (oOD-oID)/2. Providing the end rings with an outer
diameter generally equal to the rotor core outer diameter produces
favorable stress conditions in the end rings 46 when the rotor
operates at rated speed. With the end ring outer diameter generally
equal to or slightly less than the rotor core outer diameter, the
center of gravity of the end ring is sufficiently positioned toward
the axis of rotation of the rotor to reduce stress in the end
plates while providing structural integrity for the rotor core
without otherwise increasing the rotor core inertia. The end ring
outer diameter and inner diameter surface may be tapered. With
reference to FIG. 8, the end ring area ("ERA") may be provided by
the equation
ERA=[[(oOD-oID).times.w]-[w.sup.2.times.(TAN(a)+TAN(b))]]/2.
[0024] While the laminations 40 forming the rotor core may be made
from electrical steel, as is typical, the end rings 46 may be made
from a copper material or an aluminum material, or other highly
electrically conductive metal. The conductivity ("a") associated
with several commonly used materials for rotor bars and end rings
is shown below. For purposes of discussion herein, materials with a
conductivity of greater than 90% (IACS) are considered high
conductivity materials.
TABLE-US-00001 Conductivity Base Alloy Material (IACS) Aluminum
Aluminum Alloy 100.1 54% Aluminum Aluminum Alloy 130.1 55% Aluminum
Aluminum Alloy 150.1 57% Aluminum Aluminum Alloy 170.1 59% Copper
Copper Alloy C10100, 101200 101% Copper Copper Alloy C11000 101%
Copper Copper Alloy C11300, C11400, C11600 100% Copper Electrolytic
(ETP) 101% Copper Silver-bearing, 8 oz/t 101% Copper
Silver-bearing, 10 to 15 oz/t 101% Copper Silver-bearing, 25 to 30
oz/t 101% Copper Oxygen-free (OF) 101% Copper Phosphorized (DLP)
97% to 100% Copper Free-cutting (S, Te or Pb) 90% to 98% Copper
Chromium coppers 80% to 90% Copper Phosphorized (DHP) 80% to 90%
Copper Cadmium copper (1%) 80% to 90%
As mentioned before, the rotor bars may be fabricated/swaged or may
be cast. One or both of the end rings may be fabricated and/or
cast. To allow a cast end ring to be removed from a mold, the ring
outer and inner diameter surfaces may be tapered as shown in FIGS.
7 and 8. The rotor bars and end rings may be made from different
materials with different or similar conductivities. The rotor bars
and end rings may also be made from the same material.
[0025] A decrease in asynchronous slip may be achieved when the
rotor bars are made from materials with favorable conductivity
properties. For instance, for a given load and starting current,
the asynchronous slip of a LSIPM may be decreased by forming its
rotor bars from copper allows rather than from aluminum alloys.
[0026] A decrease in asynchronous slip may be achieved when the
minimum geometric cross-sectional area of the end rings ("ERA") is
greater than 0.5 times the rotor bar area ("BA") per the number of
poles (P) times a ratio of the rotor bar material conductivity to
the end ring material conductivity (.sigma..sub.RB/.sigma..sub.EM).
FIG. 9 provides a chart showing data associated with a 280 size
frame, 4 poles/3 phase LSIPM with a 6'' core length having a total
rotor bar area ("BA") of 4.0050468 in.sup.2. The bar area per pole
(BAP) was given by the equation (BAP=(BA/P)) and is shown as
1.001262. Because the rotor bars and the end rings were formed from
the same material, the ratio of the rotor bar material conductivity
to the end member material conductivity
(.sigma..sub.RB/.sigma..sub.EM) is 1. Each row of the chart of FIG.
9 corresponds to a rotor configuration comprising an end ring with
the same outer diameter and inner diameter (i.e., the same height
("h")) and a different width ("w"). The slip associated with each
configuration decreases as the ERA/BAP approaches 0.5. When ERA/BAP
approaches 1.3, slip is relatively constant. Likewise, the maximum
inertia to be synchronized increases as the ERA/BAP approaches 0.5,
and when ERA/BAP approaches 1.3, the maximum inertia to be
synchronized is relatively constant. In each case, the stator flux
was essentially a constant value with a starting current ranging
from 269 to 273 amps. For purposes of discussion herein, an
equivalent range includes variations of up to and including 2.5% of
starting current. As shown in the tabulated data of FIG. 9, a range
of ERA/BAP of about 2/3 to about 2.0 for rotor bars and end rings
having the same conductivity provides improvements in inertia
synchronization capability without significant impact on starting
current.
[0027] If end rings and bars are cast, there is a potential for
porosity which decreases conductivity. Thus, the ratio of ERA/BAP
may be selected toward the higher range to account for the decrease
in effective conductivity of the end ring due to the higher
porosity in the end ring than the rotor bar. In a similar fashion,
the ratio of the rotor bar material conductivity to the end member
material conductivity (.sigma..sub.RB/.sigma..sub.EM) provides a
factor to account for rotor bar materials and end ring materials
having different material conductivities. For instance, where the
rotor bars are made of copper and the end rings are made of
aluminum, the ratio of the conductivity of copper to the
conductivity of aluminum is 1.772 (i.e., 101/57). A factor of 1.772
can then be applied to ERA/BAP as desired. Thus, in the example of
FIG. 9, in order for the motor to synchronize a load of 74
lb-ft.sup.2, the minimum width of an aluminum end ring used with
copper rotor bars would need to be increased from 0.850 in. to
1.505 in. for the same diameter end ring. While the drawings show
end rings having the same cross-sectional geometry, one end ring
may vary dimensionally from its axially opposite end ring, for
instance, have a different width (w) dimension. However, for
purposes of a design capable of synchronizing the higher inertia as
described above, the cross sectional area ("ERA") of both end rings
should be no less than about 0.5 times greater than the rotor bar
area per the number of poles (BAP) times a ratio of the rotor bar
material conductivity to the end member material conductivity
(.sigma..sub.RB/.sigma..sub.EM), and preferably between about 2/3
and about 2 times greater than the rotor bar area per the number of
poles (BAP) times a ratio of the rotor bar material conductivity to
the end member material conductivity
(.sigma..sub.RB/.sigma..sub.EM). As is seen in the foregoing, a
LSIPM motor according to the present teachings may drive a
relatively higher inertial load from start to at least near
synchronous speed during steady state operation of the motor when
its ERA is greater than about 0.5 times the rotor bar area per the
number of poles (BAP) times a ratio of the rotor bar material
conductivity to the end member material conductivity
(.sigma..sub.RB/.sigma..sub.EM), with other variables associated
with the motor being equal or remaining constant, for instance,
starting current, starting torque, bar configuration, magnet
configuration, or other configurations of the laminations.
[0028] The performance of an LSIPM during synchronous steady state
operation may be enhanced by maximizing the saturation boundary
area and the magnet size. These considerations result in less
lamination area being available for rotor bars for a given size
motor. Providing an end ring member with a minimum geometric cross
sectional area ("ERA") that is greater than about 0.5 times the
rotor bar area per the number of poles (BAP) times a ratio of the
rotor bar material conductivity to the end member material
conductivity (.sigma..sub.RB/.sigma..sub.EM) provides improvements
in the LSIPM's ability to synchronize loads with a relatively high
inertia. For instance, a LSIPM having an end ring member with a
minimum geometric cross sectional area ("ERA") that is greater than
about 0.5 times the rotor bar area per the number of poles (BAP)
times a ratio of the rotor bar material conductivity to the end
member material conductivity (.sigma..sub.RB/.sigma..sub.EM) can
synchronize a load with an inertia that is greater than the load
the LSIPM motor can synchronize when the end ring member has a
minimum geometric cross sectional area ("ERA") that is less than
about 0.5 times the rotor bar area per the number of poles (BAP)
times a ratio of the rotor bar material conductivity to the end
member material conductivity (.sigma..sub.RB/.sigma..sub.EM), for
an equivalent range of starting current. In another aspect of the
teachings, providing a rotor cage formed from materials having
conductivity greater than other materials allows for improvements
in the LSIPM's ability to synchronize loads with a relatively high
inertia. For instance, a LSIPM having a rotor cage formed from a
material having a first conductivity can synchronize a load with an
inertia that is greater than the load the LSIPM motor can
synchronize when the rotor cage is formed from materials having a
conductivity lower than the first conductivity and the motor is
subjected to an equivalent range of starting current.
[0029] While certain embodiments have been described in detail in
the foregoing detailed description and illustrated in the
accompanying drawings, those with ordinary skill in the art will
appreciate that various modifications and alternatives to those
details could be developed in light of the overall teachings of the
disclosure. Particularly, the figures and exemplar embodiments of
the rotor laminations are intended to show illustrative examples
and not to be considered limiting in any sense. Accordingly, the
particular arrangements disclosed are meant to be illustrative only
and not limiting as to the scope of the invention which is to be
given the full breadth of the appended claims and any and all
equivalents thereof.
* * * * *